The new Range Rover is a completely different car to the last one, thank God, with the old model's antique live axles and separate chassis finally consigned to history. Also into the time capsule goes the trusty old Rover V8 engine (which Overfinch used to throw away anyway), replaced by a thoroughly modern BMW 4.4-litre V8. Since Overfinch doesn't officially recognise anything under 5 litres as actually being an engine, it immediately got the BMW block on the workbench and managed to grind away enough metal to make space for bigger pistons and a longer throw crankshaft.

The end result is a healthy five litres with a useful 100bhp more than the standard 4.4's 286bhp, though torque has risen by a comparatively modest 53lb ft to peak at 376lb ft.

As the latest Range Rover is 500kg heftier than the last one, even with the new engine tweaks, yet more help was needed to give the 580S the degree of urgency expected of an Overfinch. So attention turned to the gearbox.

Overfinch decided to rebuild the five-speed auto gearbox on the 580S and fit a special close-ratio gear set, including a lower first gear to give the big 4x4 a brisker step-off from a standing start than a standard car. The end result of both these tweaks is a 0-60 time of 7.1sec and 0-100 in 19.8sec, some nine seconds quicker than standard. The cost of the engine and gearbox mods: a smidge under ΂£20K.

Overfinch offers endless wood options for the cabin, enabling you to ditch the strange 'foundry' finish on the centre dash. The most useful mod to the interior is the highly professional seven-seater conversion (from ΂£1386) that should have been offered by Land Rover in the first place. Overfinch's body kit also works well. What hasn't been touched are the chassis (apart from a different 20in wheel design) and brakes.
Add up all the options on this test car and you stick almost ΂£45,000 on top of the standard car's price; we're looking at a ΂£105,000 Range Rover here.

Strange, then, that when I first drive the 580S I'm disappointed. Trouble is, I've driven most of the previous Overfinch cars and this is the first without the distinctive American V8 grumble of a small-block Chevy under the bonnet. It may have been a little crude in its mechanical makeup, but who cared when it dispensed colossal torque from tickover. The last Overfinch Rangey we tested was the monstrous 630R; it had 457lb ft and 400bhp, so this 580S is some way short, especially on torque.

Overfinch would love there to be more (it's what the company's all about, after all) but the problem is the gearbox. It can't cope with any more than the 376lb ft the 580S currently delivers. BMW was thinking of fitting the V8 diesel from the 7-series to the X5 (which shares the Range Rover's gearbox) but came up against the same problem. German tuners are even turning X5s into strange two-wheel-drive contraptions so they can dramatically up the engine's torque via supercharging.

Once the engine starts to spin towards the redline in second or third you can feel the extra grunt but it seems barely different to the standard car's. (The launching abilities of previous Overfinch conversions were violent enough to hurl front passengers into the boot.)

The Overfinch 580S is in many ways a terrific car, mainly because the base car is so good to start with. The 580S feels more like a factory-produced 'S' version of the Range Rover rather than an a true Overfinch to me. What it really needs is a supercharger to give it the torque it deserves. Until then the Porsche Cayenne will remain the performance SUV of the moment.